I once had something similar happen on an Asus PC 4800 Intel board when I did not put the video card all the way into the slot, but that blows on startup, not viewing the web.
Have you got another card to try ? Maybe even a PCI card to check the AGP slot ?
I just posted you this via PM
http://www.aocb77.dsl.pipex.com/6600gtcooling.jpg
but I guess you might not be interested in attaching the cooler now unfortunately. I have decided, maybe in vain hope, that the AGP bridge chip ( which runs very hot ) is causing the problems so I have stuck part of an MX 4000 heatsink on it ( the rest is being used as RAM cooling ). I think I'm clutching at straws to be honest.
Regards
Andy